Although curing of concrete under plastic sheets is a recognized technique, I have some doubts about it.

I think that the plastic sheets may trap heat coming from the exothermic reaction, which will result in important temperature rise in fresh concrete.

I am concerned about heat coming from inside and not outside.

My question is: is this a real issue?

  • $\begingroup$ Covering also keeps moisture in which aides curing. $\endgroup$ Aug 15 '20 at 18:13
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    $\begingroup$ what is the R value for your plastic? In cold climates such as Canada winter, that plastic sheet wont be enough. You may wind up throwing insulating blankets or straw to help trap that heat. Alternatively you may see a contractor tarp an area and have furnaces blowing hot air in. I have seen guys in there working in T-shirts it so warm, but at the same time having to wear a parka outside of the tarp while looking in. So the amount of heat trapped by plastic is not an issue. In hot weather you will use other methods such as wet burlap and a sprinkler system. $\endgroup$
    – Forward Ed
    Aug 15 '20 at 22:42

The generation of heat during concrete cure is a significant issue. There are whole books written about it - see for example CIRIA C766 - 'Control of cracking caused by restrained deformation in concrete'. The primary cause of the cracking it discusses is so-called 'early thermal cracking', where the heating and subsequent cooling of the concrete sets up restrained strains that gives rise to cracking.

In a simple case, imagine you cast a panel of slab as an infill between two already-hardened panels. As the concrete cures, it heats up, and wants to expand, but it has nowhere to go (it’s between two solid slab edges) so you’d think it will go into compression. However, this early in the cure, the cement matrix is really weak, and the compression that would be generated by this attempt to expand is dissipated by creep. The concrete reaches a decent strength while it's still hot, then it starts to cool down. As it cools, it tries to contract. However, it’s bonded to those adjacent slab edges, so can't actually contract, so it goes into tension (it’s now strong enough that creep doesn’t take care of it). The concrete ends up in tension, and might crack, and if it does crack (due to this effect), probably will through the full thickness of the slab (which is generally not a good thing).

However, this is a relatively complex topic, and it is not necessarily the case that cooling the concrete will improve matters. Indeed, one of the ways of mitigating thermal cracking is to insulate the concrete surface, and deliberately trap the heat in. This is because another of the mechanisms that can trigger cracking is the differential cooling through the section - the surface cools quicker than the core. If you minimise the difference between surface and core, you minimise this sort of cracking. Unfortunately, that makes the case described above (which is driven by the fall from peak temperature to ambient temperature) worse, because insulating the concrete increases that peak temperature and therefore increases the fall from peak to ambient.

The ways to address early thermal cracking are many and varied, and depend upon the details of the situation - the shape and size of the member, the arrangement of restraints, the sequence in which adjacent parts are cast, etc. One measure that works in almost all cases is to reduce the heat by using less exothermic mixes - this is one of the major benefits of pozzolanic cement substitutes in the blend. You also need to provide sufficient reinforcement to control the cracking – these effects can even yield the reinforcement if you get it wrong (and then you get big cracks). The main application of C766 is to determine how much reinforcement you need just to control these thermal effects. You can also cool the concrete, maybe by using flaked ice when you mix it rather than liquid water, or even by casting in cooling pipes (the latter tends only to be done if all else fails).

However, the heat generated, and temperatures reached, are such that a plastic sheet on the surface is substantially irrelevant – it won’t change the temperatures to any significant degree. The plastic sheet on curing concrete is normally there to keep the water in, so the surface cures properly, and making certain that the cover zone concrete is dense, sound, properly cured concrete is absolutely critical to achieve durable concrete.


When waiting for concrete to cure in conditions where it is likely to freeze then it is often covered.


When casting large machine foundations at sites in tropical conditions we always covered the concrete with sacking that was constantly kept wet to absorb the heat of the curing concrete. We never used non-absorbant covering like plastic.

  • $\begingroup$ That's not to absorb the heat - you kept it wet to prevent the surface concrete drying out before it was fully cured. $\endgroup$
    – achrn
    Aug 18 '20 at 17:29

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